1. Field of the Invention
This invention relates to a method of and apparatus for carrying out image processing on an image signal obtained from a recording sheet on which a radiation image is recorded and particularly on an image signal representing a radiation image recorded using an irradiation field stop for limiting the irradiation field of radiations.
2. Description of the Related Art
Carrying out a suitable image processing on an image signal obtained by reading a recorded radiation image and reproducing a visible image on the basis of the processed image signal have been practiced in various fields, for instance, in a radiation image recording and reproducing system using stimulable phosphor sheets disclosed in our many patent applications.
When recording a radiation image on a recording sheet, it is often desired that portions of the object not related to diagnosis or the like be prevented from being exposed to radiation, thereby preventing adverse influence of radiation on the object. Further when the object portions not related to diagnosis or the like are exposed to radiation, the radiation is scattered by such portions and the quality of the radiation image is lowered by the scattered radiation. Therefore, an irradiation filed stop is often used in order to limit the irradiation field so that the radiation are irradiated only on the necessary part of the object.
In such a case, the read-out conditions and image processing conditions are determined on the basis of the image signal components corresponding to the picture elements in the irradiation field. The irradiation field can be recognized, for instance, in the following manner as disclosed in U.S. Pat. No. 4,967,079. That is, image signal components for picture elements on each of a plurality of radial linear segments joining a predetermined point in the irradiation field and a plurality of points on the edges of the recording sheet are read out, and a prospective edge point, which is considered to be on the edge of the irradiation field, is determined for each segment, and the irradiation field is recognized as a region which is circumscribed by a line passing through the prospective edge points.
The radiation image is reproduced, for instance, on a CRT as a visible image or recorded on a photographic film as a visible image by use of a laser printer on the basis of an image signal obtained according to the read-out condition and the image processing conditions thus determined.
However in the case where the radiation image, which is the original of the visible image, is recorded by use of an irradiation field stop, the area of the reproduced image corresponding to the area outside the irradiation field is high in brightness (as reproduced on the CRT) or low in density (as recorded on the film) since the area outside the irradiation field is hardly exposed to radiation. Strong light impinging upon the eyes from the area outside the irradiation field makes the image in the irradiation field difficult to view no matter how high the quality of the image in the irradiation field is. In order to overcome such a problem, conventionally a light-shielding plate or the like is sometimes disposed on the CRT or the film to block light from the area outside the irradiation field.
However since the shape and/or size of the irradiation field differ according to the object, it is troublesome to change the position of the light-shielding plate by the shape and/or size of the irradiation field. Further it is very difficult to clearly separate the area inside the irradiation field from the area outside the irradiation field by the light-shielding plate, and the light-shielding plate sometimes covers up to the edge of the irradiation field and sometimes permits leak of light from the area outside the irradiation field. Thus, the light-shielding plate cannot satisfactorily overcome the aforesaid problem of difficulties in viewing the image in the irradiation field due to light from the area outside the irradiation field.
In the method of and apparatus for processing a radiation image disclosed in Japanese Unexamined Patent Publication No. 3(1991)-98174, the image signal components for picture elements outside the irradiation field are set to a low brightness (when a visible image is reproduced as a brightness distribution on a CRT or the like) or high density (when a visible image is reproduced as a density distribution on film or the like), generally to a minimum brightness or a maximum density, thereby obtaining a visible image free from strong light impinging upon the eyes from the area outside the irradiation field.
However generally the irradiation field cannot be always recognized precisely and an area narrower than the actual irradiation field can be sometimes mistaken for the irradiation field. In such a case, a part of the radiation image which should be observed for, for instance, diagnostic purposes, can exist in the area determined to be outside the irradiation field.
In the method and apparatus disclosed in the above identified Japanese patent publication, the image signal components for picture elements recognized to be outside the irradiation field are evenly set to a low brightness or high density, and accordingly it becomes impossible to know whether the area determined to be outside the irradiation field includes a necessary part of the radiation image.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and apparatus for carrying out image processing on an image signal of a radiation image having been recorded using an irradiation field stop, which permits recognition of whether a part of a radiation image of the object exists in the area determined to be outside the irradiation field while preventing strong light from impinging upon the eyes when viewing the reproduced visible image.
In a method of carrying out image processing on an image signal representing a radiation image which has been recorded using an irradiation field stop and has an irradiation field, the method of the present invention is characterized by the steps of
recognizing the irradiation field and
carrying out gradation inversion processing on the image signal components corresponding to the picture elements recognized to be outside the irradiation field out of the image signal components which make up an image signal for reproducing the radiation image as a visible image.
In an apparatus for carrying out image processing on an image signal representing a radiation image which has been recorded using an irradiation field stop and has an irradiation field, the apparatus of the present invention is characterized by having
a means for recognizing the irradiation field and
a gradation inversion means for carrying out gradation inversion processing on the image signal components corresponding to the picture elements recognized to be outside the irradiation field out of the image signal components which make up an image signal for reproducing the radiation image as a visible image.
In the present invention, the irradiation field may be recognized by any suitable algorithm. For example, the method of recognizing the irradiation field based on the assumption that the irradiation field is square (disclosed, for instance, in U.S. Pat. Nos. 4,851,678; 5,068,907 and 5,081,580) and the method disclosed in the aforesaid U.S. Pat. No. 4,967,079 can be employed as well as other various methods.
In accordance with the present invention, since the image signal components corresponding to the picture elements recognized to be outside the irradiation field out of the image signal components which make up an image signal for reproducing the radiation image as a visible image are subjected to gradation inversion processing, the part of the reproduced image corresponding to the area recognized to be outside the irradiation field becomes low in brightness or high in density in the whole. Accordingly the problem that strong light impinges upon the eyes when viewing the reproduced visible image can be avoided. Further even when an area narrower than the actual irradiation field is mistaken for the irradiation field, whether the area determined to be outside the irradiation field includes a necessary part of the radiation image can be known.
Thus the fear that the area which was cut as being outside the irradiation field may include important information, which has been inherent to the conventional image processing, can be overcome.